LMH6505 datasheet(16/20 Pages) NSC

16 / 20 page

Application Information (Continued)

USING THE LMH6505 IN AGC APPLICATIONS

In AGC applications, the control loop forces the LMH6505 to

have a fixed output amplitude. The input amplitude will vary

over a wide range and this can be the issue that limits

dynamic range. At high input amplitudes, the distortion due

to the input buffer driving R

G may exceed that which is

produced by the output amplifier driving the load. In the plot,

THD vs. Gain, total harmonic distortion (THD) is plotted over

a gain range of nearly 35 dB for a fixed output amplitude of

0.25 V

in the specified configuration, R

=1k

Ω,

G = 100

Ω. When the gain is adjusted to −15 dB (i.e. 35 dB

down from A

VMAX), the input amplitude would be 1.41 VPP

and we can see the distortion is at its worst at this gain. If the

output amplitude of the AGC were to be raised above 0.25

PP, the input amplitudes for gains 40 dB down from AVMAX

would be even higher and the distortion would degrade

further. It is for this reason that we recommend lower output

amplitudes if wide gain ranges are desired. Using a post-

amp like the LMH6714/LMH6720/LMH6722 family or the

LMH6702 would be the best way to preserve dynamic range

and yield output amplitudes much higher than 100 mV

PP.

Another way of addressing distortion performance and its

limitations on dynamic range, would be to raise the value of

G. Just like any other high-speed amplifier, by increasing

the load resistance, and therefore decreasing the demanded

load current, the distortion performance will be improved in

most cases. With an increased R

G,RF will also have to be

increased to keep the same A

VMAX and this will decrease the

overall bandwidth. It may be possible to insert a series RC

combination across R

F in order to counteract the negative

effect on BW when a large R

F is used.

AUTOMATIC GAIN CONTROL (AGC) #1

Fast Response AGC Loop

The AGC circuit shown in Figure 9 will correcta6dB input

amplitude step in 100 ns. The circuit includes a two op amp

precision rectifier amplitude detector (U1 and U2), and an

integrator (U3) to provide high loop gain at low frequencies.

The output amplitude is set by R

9. The following are some

suggestions for building fast AGC loops: Precision rectifiers

work best with large output signals. Accuracy is improved by

blocking DC offsets, as shown in Figure 9.

20171058

FIGURE 8. Digital Gain Control

LMH6505 Datasheet(PDF) 16 Page - National Semiconductor (TI)

LMH6505 Datasheet(HTML) 16 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL